(a) Field of the Invention
The present invention relates to a Device to Device (D2D) communication method in which a terminal directly communicates with another terminal without passing through a base station, and an apparatus that supports the method.
(b) Description of the Related Art
A 3rd Generation Partnership Project (3GPP) standardization organization performs Rel-12 standardization for Device-to-Device (D2D) communication (or Sidelink transmission), which is direct communication between terminals based on a Long-Term Evolution (LTE) specification. D2D communication technology supports data being directly transmitted to an adjacent terminal without passing through a base station.
A D2D communication method, considered in 3GPP, includes mode 1 communication that receives allocation of a D2D transmitting resource by a base station or a repeater (or a relay station) and mode 2 communication in which a terminal directly selects a transmitting resource. Mode 1 communication may be used only by an in-coverage terminal, and mode 2 communication may be used by all of an in-coverage terminal, a partial-coverage terminal, and an out-of-coverage terminal. Here, a reference of in-coverage may include a case of satisfying a cell selection condition based on reference signal received power and/or reference signal received quality. The in-coverage terminal is a terminal that is located within cell coverage. The partial-coverage terminal is a terminal that receives only a synchronization signal and system information of a base station without existing within cell coverage. The out-of-coverage terminal is a terminal outside cell coverage that is not synchronized with a base station within a cell and that does not receive system information.